The Purple Wartyback (Cyclonaias tuberculata) is a medium-sized mussel with numerous bumps (pustules) on the outer surface and a purple inner shell (nacre). It was formerly common in the central and upper portions of the Illinois River, but it has not been collected alive in the river since 1912. Because of its nacre color, the purple wartyback had no value in the button industry. (Conservation Status: State Threatened)